The present disclosure relates to venting assemblies for dip coating apparatuses and related processes. It finds particular application in conjunction with the production of photosensitive members such as drum photoreceptors or layers thereof, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Electrostatographic imaging systems, which are well known, involve the formation and development of electrostatic latent images on an imaging surface of an electrostatographic or photoreceptor. Xerographic photoreceptors can be prepared in either a single-layer or a multilayer configuration. Depending on the application, the photoreceptors can be prepared in several forms, such as flexible belts, cylindrical drums, plates, etc. Belts are usually prepared on polymer substrates, poly(ethylene terephthalate) being the most common. For drums, the substrate is typically a metal cylinder. Usually, hollow aluminum cylinders are widely used in low- and mid-volume applications. The drum configuration, however, has certain process limitations for high-volume and color applications.
Photoreceptors are prepared by the sequential application of various layers (i.e., charge generating layer, charge transport layer, etc.) onto the outer surface of a polymer or drum substrate. Many coating techniques (i.e., spraying, spinning, extrusion, dipping, blade coating, roll coating, etc.) may be utilized to produce these layer(s). Vapor deposition may also be used for metallization and application of some pigments.
Most layers are coated from solutions or dispersions in organic solvents which produce solvent vapors. The choice of solvent is determined by such factors as materials solubility, evaporation rates, surface tension, toxicity, and environmental regulations. Commonly used solvent classes are alcohols, aromatics, esters, ethers, ketones, and nitrites. Because rapid solvent evaporation rates are desirable, low boiling solvents are preferred. Nevertheless, high boiling solvents such as toluene can be successfully used for some applications. In special cases, aqueous solutions or dispersions can also be used.
For the production of drum or other cylindrical photoreceptors, dip coating of one or more of the layers can be utilized. In this technique, a drum is pushed or lowered through an annulus into a bath of a coating solution to produce the desired layer(s). A related technology is dip coating where the drum substrate is dipped into a coating vessel such as a dip tube containing a bath of a coating solution and then withdrawn or pulled at a specified rate. The withdrawn or pulled drum substrate carries a thin coating of the material from the bath. The liquid coating is then dried to form a coating layer. Continuous coatings of successive drums are possible in this process.
Coating vessels used in such dip coating processes have various shapes and generally consists of a bottom, an open top and a cylindrically shaped vertical interior wall having a diameter greater than the diameter of the drum to be coated. Optionally, the coating vessel may contain a mandrel adapted to maintain the outer surface of the drum in a concentric relationship with the vertical interior wall of the cylindrical coating vessel while the drum is dipped or immersed in the coating solution. The liquid coating material in the bath may be stationary or flowing, such as circulating upwardly in the coating vessel from an inlet at the bottom of the coating vessel and allowed to overflow from the bath into an overflow tank. If desired, the coating may be continuously fed into the bottom of the coating vessel and allowed to continuously overflow from the coating vessel. The overflowing coating liquid is collected in the tank and recycled to the coating bath.
Examples of such dip coating processes and apparatuses discussed above are set forth in U.S. Pat. Nos. 4,620,996; 5,334,246; 5,681,391; 5,693,372; 5,720,815; 5,725,667; and, 6,207,337, the disclosures thereof are incorporated herein by reference in their entirety. The appropriate components and processes of these patents may be selected for the apparatuses and processes further disclosed herein.
There are many coating defects produced by dip coating processes and other coating techniques that may degrade or otherwise deleteriously affect the layers of the photoreceptor. Some defects that have been described in the literature are bloom, blush, bubbles, chatter marks, cracking, cratering, crazing, haze, mottle, orange peel, particles, repellencies, scratches, streaks, voids, etc. Drying-related defects can frequently be reduced by the judicious use of surfactants to control the surface tension. However, surfactants cannot cure all defects and in certain instances, can lead to other problems.
Additionally, the uniformity of the coating or film contributes to the electrophotographic characteristics of a photosensitive layer of an electrophotographic photosensitive member. Consequently, it is important to remove the unevenness of the coating layer(s).
In a conventional dip coating system, a vent system is used to remove solvent vapors from the dipping tank, and specifically, above the coating liquid through one or more vents along the side of the tank. This approach however, is only satisfactory since there often remains some degree of non-uniformity in the finished photoreceptors. The level of non-uniformity is within specification for many layer(s) of the products, but will not satisfy the stricter uniformity requirements for certain high grade photoreceptors.
Accordingly, a need exists for a venting assembly and method which avoids the problems associated with coating defects, and particularly non-uniformity of the resulting coating(s). Furthermore, there is a continuing need for an improved system for coating electrophotographic imaging members.